FIG. 1 is a diagram of a network structure of an E-UMTS (evolved universal mobile telecommunications system) which is one of the fourth generation wireless communication systems. The E-UMTS is the system evolved from a conventional UMTS system. And, the basic standardization is ongoing by 3GPP. Besides, the E-UMTS can be also called LTE (long term evolution).
E-UMTS network can be mainly divided into E-UTRAN (evolved UTRAN) and EPC (evolved packet core). The E-UTRAN consists of a user equipment (hereinafter abbreviated UE), a base station (hereinafter called eNode B) and an access gateway (hereinafter abbreviated AG and expressible as MME (mobility management entity)/UPE (user plane entity)) located at an end point of the network to be externally connected to an external network. The AG can be divided into one part responsible for user traffic processing and the other part for processing control traffic. In this case, the AG for new user traffic processing and the AG for processing control traffic can communicate with each other using a new interface. At least one cell can exist at a single eNode B. Between eNode Bs, an interface for user or control traffic transmission is usable. And, the EPC can consist of a node for user registrations of the AG and other UE and the like.
Interface for discriminating the E-UTRAN or the EPC is available. A plurality of nodes are connectible together via an interface S1 between the eNode B and the AG (many to many). In a meshed structure, the eNode Bs are connected via an interface X2 and the interface X2 always exists between the eNode Bs adjacent to each other.
Radio protocol layers between a user equipment and a network can be divided into L1 (first layer), L2 (second layer) and L3 (third layer) based on three lower layers of the open system interconnection (OSI) reference model widely known in the field of communication systems. A physical layer of the first layer provides an information transfer service using a physical channel, and a radio resource control (hereinafter abbreviated RRC) located on the third layer plays a role in controlling radio resources between the user equipment and the network. For this, the RRC layers exchange RRC messages between the user equipment and the network. In the E-UTRAN, the RRC layer is located at the eNode B.
FIG. 2 is a diagram of a radio protocol layer structure between a user equipment and E-UTRAN (UMTS terrestrial radio access network) based on the radio access network specification of the 3GPP that is the third generation radio communication standardization organization. Referring to FIG. 2, a radio protocol layer structure horizontally consists of a physical layer, a data link layer and a network layer. And the radio protocol layer structure vertically consists of a user plane for data information transfer and a control plane for control signal forwarding (signaling). The radio protocol layers shown in FIG. 2 can be divided into L1 (first layer), L2 (second layer) and L3 (third layer) based on three lower layers of the open system interconnection (OSI) reference model widely known in the field of communication systems.
The respective layers of the control and user planes in the radio protocol layer structure shown in FIG. 2 are explained as follows. First of all, a physical layer of the first layer provides an upper layer with an information transfer service using a physical channel. The physical layer is connected to a medium access control layer on an upper layer via a transport channel. And, data is transported between the medium access control layer and the physical layer via the transport channel. Moreover, data are transported via the physical channel between different physical layers, i.e., between a physical layer of a transmitting side and a physical layer of a receiving side.
A medium access control (hereinafter abbreviated ‘MAC’) of a second layer provides a radio link control layer of an upper layer via a logical channel. The radio link control (hereinafter abbreviated RLC) of the second layer supports a reliable data transfer. A function of the RLC layer can be implemented as a function block within the MAC. If so, the RLC layer may not exist. PDCP layer of the second layer performs a header compression function for reducing a size of an IP packet header containing relatively large and unnecessary control information to efficiently transmit such an IP packet as IPv4 and IPv6 in a radio section having a narrow bandwidth. The PDCP layer in E-UTRAN is located at the AG.
A radio resource control (hereinafter abbreviated RRC) layer located at a most upper part of the third layer is defined in the control plane only and is responsible for controlling a logical channel, a transport channel and a physical channel in association with configuration, reconfiguration and release of radio bearers (hereinafter abbreviated RBs). In this case, each of the RBs means a service provided by the second layer for the data transfer between the user equipment and the E-UTRAN.
A unit of data transferred from the respective layers of the radio protocol layer structure is called a name differing in the corresponding layer, which is called a service data unit (hereinafter abbreviated SDU). And, a basic unit used by a protocol to transfer data to another layer is called a protocol data unit (hereinafter abbreviated PDU). In the following description, data transferred within or between the layers in the radio access protocol structure of the present invention means a data block of such a prescribed unit as the SDU and the PDU.
The RLC layer is explained in detail as follows. First of all, basic functions of the RLC layer include the QoS (quality of service), security of each RB and the corresponding data transfer. Since the RB service is the service provided to an upper layer by the second layer of the radio protocol, the whole second layer affects the QoS of the RB. Particularly, the RLC has bigger influence. The RLC provides an independent RLC entity to each RB to secure the intrinsic QoS of the corresponding RB and provides three kinds of RLC modes including a transparent mode (hereinafter abbreviated TM), an unacknowledged mode hereinafter abbreviated UM) and an acknowledged mode (hereinafter abbreviated AM) to support various kinds of QoS. Since each of the three kinds of RLC modes differs in supported QoS, there exists a difference in an operational method. And, its detailed functions differ as well. In the following description, the respective operational modes of the RLC are explained.
TM RLC is the mode that no overhead is attached to SDU of a RLC layer (hereinafter named RLC SDU) delivered from an upper layer in generating a PDU at a RLC layer (hereinafter named RLC PDU). In particular, since RLC enables SDU to be transparently transmitted, it is called TM RLC. Due to the above characteristics, the TM RLC plays the following function in the user plane or the control plane. Since a data processing time is short within a RLC of the user plane and an overhead does not exist within a RLC of the control plane, the TM RLC is responsible for the transfer of a RRC message from an unspecific user equipment in uplink or is responsible for a transfer of a RRC message broadcasted to all user equipments within a cell in downlink.
Unlike the transparent mode, a mode for adding an overhead in RLC is called a non-transparent mode. And, the non-transparent mode can be classified into an unacknowledged mode (UM) of providing non-acknowledgement for transmitted data and an acknowledged mode of providing acknowledgement. UM RLC transmits each PDU by attaching a head including a sequence number to the corresponding PDU, thereby enabling a receiving side to know which PDU is lost in the course of transmission.
Owing to the above function, the UM RLC is responsible for transmission of broadcast/multicast data or real-time transmission of packet data such as voice (e.g., VoIP) or streaming of a packet service domain (hereinafter abbreviated PS domain) in a user plane. And, the UM RLC is responsible for transmission of a RRC message, which does not need acknowledgement among RRC messages to be transmitted to a specific user equipment or a specific user equipment group within a cell in a control plane.
In case that the UM RLC is used for a voice service, a size of data of the voice service amounts to 100-200 bits. An overhead occupies about 20 bits in a header, and as mentioned in the foregoing description, includes a sequence number field and various control fields (e.g., frame information field, extension field, etc.) which will be explained later.
AM RLC, which is one of the non-transparent mode, constructs a PDU by attaching a PDU header including a sequence number in the same manner as the UM RLC. Unlike the UM RLC, the AM RLC has a big difference in that a receiving side makes acknowledgement for a PDU transmitted by a transmitting side. In the AM RLC, the reason why the receiving side makes the acknowledgement is because the receiving side makes a request for the transmitting side to retransmit the PDU which is failed to be received by the receiving side. Thus, the retransmission function is the major characteristic of the AM RLC. Therefore, the object of the AM RLC is to secure error-free data transmission through the retransmission. Due to such an object, the AM RLC is responsible for non-real-time transmission of packet data such as TCP/IP of a packet service (PS) domain in a user plane. And, the AM RLC is responsible for transmission of a RRC message, for which acknowledgement is mandatory, among RRC messages transmitted to a specific user equipment within a cell in a control plane.
In aspect of directionality, both of the TM RLC and the UM RLC are used for uni-directional communication, whereas the AM RLC is used for bi-directional communication due to the feedback from a receiving side. Since the bi-directional communication is mainly used for point-to-point communication, the AM RLC uses a dedicated logical channel only. There is a difference in aspect of structure. The TM or UM RLC is configured in a manner that a single RLC entity includes a single structure for either transmission or reception, whereas the AM RLC is configured in a manner that both transmission and reception structures exist within a single RLC entity.
The AM RLC includes a retransmission buffer for retransmission management as well as a transceiving buffer. And, the AM RLC performs various schemes including a use of a transceiving window for a flow control, a use of polling for a transmitting side to make a request for status information to a receiving side of a peer RLC entity, a use of status information report (status report) for a receiving side to make a report of its buffer status to a transmitting side of a RLC entity, a use of status information PDU (status PDU) for carrying status information, a use of piggyback for inserting a status information PDU in a data area of a PDU to raise efficiency of data transmission and the like. To support these functions, the AM RLC needs various protocol parameters, state variables and a timer. Thus, the PDUs used for data transmission control in a RLC layer, such as status information report, are called control PDUs. And, PDUs for carrying user data corresponding to a payload on a data area part are called data PDUs.
The AM RLC layer performs relevant operations to enable the entire data, which are transmitted to a receiving side by a transmitting side, to be successfully received by the receiving side. For instance, a transmitting side is able to receive transceiving confirmation information (e.g., RLC status report) from the relieving side, which is a status information report for data transmitted to a receiving side. And, the transmitting side is able to perform a retransmission for the data identified as not received by the receiving side via the transceiving confirmation information.
Yet, the transceiving confirmation information transmitted by the receiving side may be lost on a radio channel in the course of being transmitted to the transmitting side. If this situation is possible, the transmitting side can aggressively make a request for the receiving side to transmit the transceiving confirmation information. The procedure associated with the request by the transmitting side and a response to the request made by the receiving side is called a polling procedure.
In the course of the polling procedure, the transmitting side provides a field for indicating a presence or non-presence of a request for a transmission of transceiving confirmation information in a header of RLC PDU, which is transmitted to the receiving side by the transmitting side itself, thereby the transmitting can make a polling request to the receiving side.